N-Methylation Effects on the Coordination Chemistry of Cyclic Triamines with Divalent Transition Metals and Their CoII Dioxygen Carriers

The thermodynamics of complex formation of Co-II and Cd-II ions with the triaza macrocyclic ligand 1,4,7-triazacyclononane (tacn) and its N-methylated derivative 1,4,7-trimethyl-1,4,7-triazacyclononane (Me(3)tacn) has been studied in dimethyl sulfoxide (DMSO) at 298.1 K and in an ionic medium (0.1 M Et4NCIO4) by means of potentiometric, UV/Vis, calorimetric and FT-IR techniques. The results are discussed by taking into account electronic and steric effects as well as solvation of the species concerned. Computational methods based on density functional theory (DFT) have been used to obtain structural information about the ligands and their complexes in order to provide further, independent insights into the effect of N-methylation on the coordination affinity of the ligands towards the metal ions. The computational suggestions are of great help to correlate steric effects and thermodynamic results. The kinetics of dioxygen uptake for the formation of the Co(tacn)(2)O-2 superoxo adduct has also been studied by means of UV/Vis measurements.